This invention relates to a method for fabricating thick film alumina structures used in high frequency, low loss applications and a structure resulting therefrom. More particularly, the invention is directed to a co-fire process for fabricating, or laminating, thick film structures to one another by printing a conductor layer between two thick film circuits, drying the layer, and co-firing the combined structure. This process is particularly useful in fabricating high frequency (e.g. 10 gigabit) fiber optic transmitters.
While the invention is particularly directed to the art of fabricating thick film alumina structures used in high frequency, low loss applications, and will be thus described with specific reference thereto, it will be appreciated that the invention may have usefulness in other fields and applications. For example, principles of the invention may be used in any application where an alternative to LTCC (low temperature, co-fired ceramic) processes is desired.
By way of background, within the ceramics industry, there are primarily two technologies used for fabricating multi-layer ceramic structures: thick film (on 96% alumina) processes and LTCC (low temperature co-fired ceramic) processes. Thick film processes have historically been the dominant technology.
In this regard, thick film processes have many advantages over LTCC processes. For example, thick film processes are traditionally conducted by established manufacturers with established supply chains. As such, there is lower cost in implementing these processes. From a performance standpoint, thick film processes result in lower loss for high frequency structures. In addition, thick film processes result in structures having greater thermal conductivity.
There are also disadvantages to the use of thick film processes. For multi-layer circuits used in high frequency applications, thick film processes result in poor performance of the structure. Additionally, since dielectrics can only be printed, thick film processes are limited to using a thin dielectric layer. The thin dielectric layer limits the types of high frequency structures that can actually be built.
Likewise, LTCC processes have advantages and disadvantages. For example, LTCC processes have at least some performance advantages over other technologies; however, LTCC processes require a large capital investment on the part of the manufacturer. As to disadvantages in performance, LTCC processes typically result in higher loss ratios than thick film technology for the structure fabricated. Moreover, LTCC tape is typically not available in comparable alumina thickness, thereby requiring more layers of tape to realize an equivalent thickness in alumina. Still further, LTCC processes typically result in shrinkage of fabricated elements. This shrinkage is, of course, not desired and must be compensated for in the design.
Accordingly, it is desirable to implement a process that realizes the benefits of both thick film processes and LTCC processes. However, no such process was heretofore available.
The present invention contemplates a new and improved process for fabricating thick film alumina structures used in high frequency, low loss applications that resolves the above-referenced difficulties and others.